Various isotopes including fissionable materials are contained in fuel rods and radioactive waste that are handled in nuclear fuel cycles for nuclear power generation. Inspecting these in a nondestructive manner and visualizing their spatial distribution are important in realizing safe and efficient nuclear fuel cycles. By way of example, with respect to cesium, there is Cs-133, which is a stable isotope with an atomic mass number of 133, and there is Cs-137, which is a radioactive nuclide with an atomic mass number of 137, where the handling of the latter is strictly controlled by law. In radioactive waste treatment, by quickly identifying them, costs associated with underground disposal can be reduced dramatically. The realization of a technique for identifying isotopes and visualizing the spatial distribution thereof is therefore strongly desired.
In addition, in transporting nuclear fuel materials, measuring, in a nondestructive manner and from outside of a container, fissionable materials, such as uranium, etc., nuclear fuel materials, explosives, and materials from which they are made concealed inside the container is extremely important for purposes of strictly restricting/controlling transportation of nuclear materials, in preventing terrorism, etc., involving explosives, and in realizing a safe and secure society.
Currently, at some nuclear fuel cycle facilities, seaports, airports, etc., X-ray transmission image-based internal geometry measurement and nondestructive inspection of fissionable materials, nuclear fuel materials, explosives, etc., are performed using large-scale X-ray inspection equipment that inspects fuel rods and containers in their entirety, or prompt gamma ray analyzers that use a neutron generator, etc. In X-ray inspection, a hard high-energy bremsstrahlung X-ray is used, and while there is an advantage in that a clear transmission image is obtained, the material cannot be identified. Prompt gamma ray analysis by way of neutron radiation allows for material identification and isotope identification, but has poor spatial resolution, and its spatial resolution is insufficient for interior visualization. The term material identification as used herein refers to element identification, that is, to the identification of atoms. This may be done by observing the electron state around the nucleus, and may be observed with relative ease by means of X-rays, etc. The term isotope identification refers to the identification of an isotope, that is, of a nucleus with a different number of neutrons, with respect to the protons and neutrons contained in the nucleus, and may be observed by detecting gamma rays.
With respect to material inspection of the interiors of import container cargos and suit cases, there is proposed an isotope identification method based on nuclear resonance fluorescence (NRF) using bremsstrahlung X-rays (Patent Document 1). The nucleus of an isotope has, depending on the number of protons and neutrons, which are constituent elements thereof, a natural frequency (excitation level). When the isotope is irradiated with a photon having an energy that matches this frequency, the isotope absorbs the photon, and a fluorescent photon is thereafter generated upon deexcitation, and this is referred to as NRF. Isotope identification may be carried out by observing NRF gamma rays with a radiation detector. Since NRF gamma rays have an energy of several MeV and are capable of passing through an iron plate of approximately 10 mm, isotope identification for and the spatial distribution of a material sealed inside a container, etc., may be measured in a nondestructive manner. The concept of the method is shown in FIG. 1.
A sample 2 is irradiated with photon beams 1 of X-rays, photons, etc. An isotope 3 of interest is contained in the sample 2. It is noted that the sample 2 may be shielded in some cases, however the shield is omitted here. The isotope 3 absorbs the photon beams 1, and emits an NRF gamma ray 4 which is detected by a radiation detector 6. The other photons are scattered by the other atoms within the sample to become scattered X-rays 5, and either exit the system or are detected by the radiation detector 6. A portion of the photon beams 1 that has been transmitted is measured at a photon intensity monitor 7. By scanning the photon beams 1 or by moving the sample 2, the spatial distribution of isotopes is measured.
With respect to the above-mentioned inspection, analysis, or treatment process, there is proposed a method in which quasi-monochromatic photons are generated through laser-Compton scattering (LCS, later discussed) instead of bremsstrahlung X-rays, and in which these are used for isotope detection. It is possible to generate LCS photons in the photon range of several MeV by irradiating a high-energy electron beam, which is generated by an electron accelerator, etc., with high-intensity laser.
As in ordinary Compton scattering, LCS is an interaction between electrons and photons, but is characterized in that the energy of the electrons is high, and that laser is used as photons. Photons generated by this method have the following characteristics: the photons are emitted within an extremely narrow solid angle and have high directionality comparable to synchrotron radiation; the photons may be made quasi-monochromatic by means of a collimator while at the same time reducing the energy spread (making them quasi-monochromatic) (Equation (1)) since there is a correlation between scattering angles of photons and energy; LCS photons with a high degree of polarization may be obtained since the polarization of the laser is preserved as is in the scattered photons (Equation (6), later discussed); and so forth.
The principles of LCS are represented in FIG. 2, and the relationship between energy Eγ of LCS photons and energies of electrons and laser light through Equation (1). In Equation (1), Ee represents the energy of electrons, and EL the energy of laser light.
                                          E            γ                    =                                                    E                L                            ⁡                              (                                  1                  -                                      β                    ⁢                                                                                  ⁢                                          cos                      ⁡                                              [                                                  θ                          1                                                ]                                                                                            )                                                    1              -                              β                ⁢                                                                  ⁢                                  cos                  ⁡                                      [                                          θ                      2                                        ]                                                              +                                                                    E                    L                                    ⁡                                      (                                          1                      -                                              β                        ⁢                                                                                                  ⁢                                                  cos                          ⁡                                                      [                                                                                          θ                                2                                                            -                                                              θ                                1                                                                                      ]                                                                                                                )                                                                    E                  e                                                                    ⁢                                  ⁢                                            where              ⁢                                                          ⁢              β                        =                                          1                -                                  γ                  2                                                              ,                      γ            =                                          E                e                            0.511                                                          (        1        )            
The relationship between energy and scattering angle of LCS photons with respect to cases where electrons with an energy of 641 MeV are irradiated with lasers whose wavelengths are 1064 nm and 1550 nm is shown in FIG. 3. By restricting scattering angle θ2, it is possible to obtain photons with the desired energy and energy width. Specifically, the scattering angle is restricted by positioning a collimator in which a narrow hole is opened in lead, etc., along the beam axis. The energy width is ordinarily on the order of several %, and these are referred to as quasi-monochromatic photons.
An isotope's reaction cross-section σD(E) is given by Equation (2). Resonance width Γ is broadened due to Doppler broadening as given by Equation (3). However, the width of Δ is extremely narrow, and is ordinarily on the order several hundred meV.
                                          σ            D                    ⁡                      (            E            )                          =                                                            π                                  3                  2                                            (                                                ℏ                  ⁢                                                                          ⁢                  c                                E                            )                        2                    ⁢                                                    2                ⁢                                  I                  1                                            +              1                                                      2                ⁢                                  I                  0                                            +              1                                ⁢                      Γ            Δ                    ⁢                      exp            [                          -                                                (                                                            E                      -                                              E                        res                                                              Δ                                    )                                2                                      ]                                              (        2        )                                Δ        =                              E            res                    ⁢                                                    2                ⁢                                  kT                  eff                                                            mc                2                                                                        (        3        )            
Thus, in order to cause NRF efficiently, it is preferable that the excitation photon have a narrow energy spectrum that synchronizes with the natural frequency of the isotope. In the equations above, ℏ represents the Planck constant/2π, c the speed of light, E the photon energy, I0 and I1 the total angular momenta in the ground state and the excited state, respectively, Eres the resonance energy, Γ the resonance energy width, k the Boltzmann constant, Teff the effective temperature of a nucleus, and m the rest mass energy of an electron.
Since the energy spread of LCS photons can be narrowed to or below several %, it is possible to increase the signal-to-noise ratio (S/N) by reducing background noise (noise), which is advantageous to the method using bremsstrahlung X-rays. Thus, measuring methods that employ an LCS photon beam are superior in many aspects, such as precision, time, reliability, safety, etc., over cases where a bremsstrahlung X-ray is used.
In Non-Patent Document 1, there is proposed a method that uses LCS photons in an isotope identification method employing NRF. In Non-Patent Document 2, there is reported a method in which an Energy Recovery Linac (ERL), which is a next-generation electron accelerator, is combined with a state of the art high-power mode-locked fiber laser and a super cavity that accumulates pulsed laser, thereby generating LCS photons that are far more intense (approximately 108 times so) than existing LCS photons. It is indicated that the abundance of long-lived nuclides within radioactive waste can thus be detected in a few seconds. Non-Patent Documents 3 and 4 contain reports regarding the detection of an NRF gamma ray with an energy of 5512 keV generated from lead-208, which is an isotope of lead, and imaging based thereon, as well as the detection of a 4439 keV NRF gamma ray from carbon-12, and material identification based thereon. The lead-208 sample was carefully concealed within an iron box with a thickness of 1.5 cm.
The nuclear excitation levels of carbon-12 and lead-208 are shown in FIG. 4. An LCS photon beam with a narrow energy width is used to excite nuclei. With the exception of hydrogen, there exists a unique excited state for each nucleus. When photons are emitted at the level (e.g., 5512 keV) of the nuclide to be measured (e.g., lead-208), the 5512 keV photons are absorbed by lead-208. In the process of cooling from the excited state, lead-208 emits NRF gamma rays that are equivalent to the excitation energy. By detecting these, it is possible to detect lead-208. LCS photon beams of the desired energy are generated by irradiating, with a laser with a wavelength of 1064 nm, 560 MeV electrons with respect to lead-208, and 510 MeV electrons with respect to carbon-12.
As a method for accurately analyzing elements contained within a substance, there is X-ray fluorescence analysis. Since X-ray fluorescence is low in energy, substances concealed inside containers cannot be measured. Further, since element analysis is carried out by utilizing the fact X-rays generated due to the structures of atoms, that is, due to electron transition (characteristic X-rays), represent states unique to the atoms, while elements may be identified, isotopes may not.
There is proposed a method in which a sample is irradiated with a high-energy gamma ray to induce a (γ, n) reaction with respect to the isotope of interest, thereby producing nuclear isomers, and in which isotope identification is carried out using the deexcitation gamma ray thereof (Patent Document 2).